In long-distance transmissions with higher speeds of 10 Gb/s or greater, the phase state, namely, change in the frequency (wavelength) over time (chirp) is pivotal, not to mention the contrast between the on state and the off state (extinction ratio) generated by intensity modulation.
Particularly, it is desirable to set the frequency difference between the signal on state and the signal off state (frequency modulation amplitude) generated by intensity modulation so as to reduce waveform distortions caused by the wavelength dispersion experienced by optical signals propagating through an optical fiber.
Furthermore, optical fiber communication systems have been proposed, which include an optical signal source configured to generate partially frequency modulated signals and a wavelength filter configured to convert the partially frequency modulated signals into substantially amplitude modulated signals, in order to compensate for scattering in an optical fiber.
Such optical fiber communication systems employ various types of optical signal sources for generating partially frequency modulated signals, such as a directly modulated laser (see FIG. 17A), an external phase modulator (MOD) (see FIG. 17B), and a tunable laser (see FIG. 17C). See D. Mahgerefteh et al., “Error-free 250 km transmission in standard fiber using compact 10 Gbit/s chirp-managed directly modulated lasers (CML) at 1550 nm”, ELECTRONICS LETTERS, 28 Apr. 2005, Vol. 41, No. 9; and Japanese Translation of PCT International Application No. 2006-516075, the entire contents of which are incorporated herein by reference.